Foraging in the Kuiper belt on the outskirts of the solar system, the dwarf planet Makemake is no stranger to icy conditions. Farther afield lies Eris, a white world that is coated in frost. Closer to the sun than both of them sits Pluto, which gets just enough heat to vaporise the rime, granting the ruling dwarf a thin atmosphere of nitrogen gas.

Makemake, meanwhile, feels a bit cheated. It is big enough and just about close enough to the sun that everyone expected it to have an atmosphere like Pluto's. But a fresh look has revealed the dwarf planet has only hints of a patchy, threadbare cloak.

Astronomers now think Makemake is less dense than Pluto, so it could not hold on to its atmosphere. These new details show that dwarf planets are surprisingly diverse, and may help us better understand how atmospheres form and evolve on rocky worlds.

Named after a Rapa Nui fertility god, Makemake appears to be about two-thirds the diameter of Pluto, so it should be massive enough to be almost round. Chemical clues gathered from its light hint that the world is covered in methane ice with a dash of nitrogen. Based on size and chemistry, Makemake should have an atmosphere.

Watching for winks

But the distant body is tough to resolve on its own. So last year a team led by Jose-Luis Ortiz of the Institute of Astrophysics of Andalusia in Granada, Spain, used a rare type of eclipse to tease out new details. Telescopes across South America watched as Makemake passed in front of a background star, causing it to wink out briefly.

Changes in starlight due to these stellar occultations can reveal an object's size, how reflective it is and whether it has an atmosphere.

We know from previous occultations by Pluto that it has a thin nitrogen atmosphere and is not very reflective. Occultations by Eris reveal that it is almost the same size as Pluto, but is much shinier. This is probably because its atmosphere froze and coated the surface in fresh ice.

Makemake falls in between. Its occultation showed that it reflects 77 percent of incoming light and has no significant atmosphere. The team concludes that the dwarf planet must be such a lightweight that it lost the bulk of its nitrogen in its youth.

"We believe that Makemake probably had plenty of nitrogen ice in the ancient past, like Pluto and Eris, but because Makemake is not very massive, its gravity could not retain the gas," says Ortiz. The nitrogen sublimated &ndash changed from ice directly into a gas – over aeons, leaving mostly methane behind.

Tufts of methane

The middling shininess matches previous observations of heat coming from Makemake's surface, which revealed dark spots that absorb more sunlight. It is possible that methane is sublimating into tufts of the gas that hover over these warm dark patches, but they may be in the wrong places for us to see them during an occultation.

Francis Nimmo of the University of California, Santa Cruz, who was not involved in the study, agrees with the team's findings. In the very long term, he adds, the family of dwarf planets will evolve into worlds that are very different from each other. "When the sun becomes a red giant... Kuiper belt objects will become warmer and develop much thicker atmospheres. Pluto's atmosphere will then be completely different from Makemake's, because it managed to hold onto a different set of gases."

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